EPLD CHIP WITH HYBRID ARCHITECTURE OPTIMIZED FOR BOTH SPEED AND FLEXIBILITY

Abstract: A hybrid EPLD architecture has multiple first blocks (FB2-FB11) each including a first type programmable AND array and multiple first type macrocells which are complex in structure and highly configurable; and multiple second blocks (FFB0-FFB1) each including a second type programmable AND array having fewer input lines and product term output lines than does the first type AND array, and multiple second type macrocells which have fewer logic gates than do the first type macrocells. The EPLD has a programmable interconnect matrix (UIM) for interconnecting all the blocks.

Description: EPLD CHIP WITH HYBRID ARCHITECTURE OPTIMIZED FOR BOTH SPEED AND FLEXIBILITY FIELD OF THE INVENTION This invention relates generally to programmable logic devices, and more particularly to a hybrid architecture including both high-speed blocks and high-flexibility blocks for the logical functions in Programmable Array Logic-based EPLDS. BACKGROUND OF THE INVENTION Basic PLDs (programmable logic devices) are based on a programmable logic array, normally composed of a specific number of input lines connected through a fixed or programmable array to the input terminals of a set of AND logic gates, the output terminals of which are in turn connected to the input terminals of a fixed or programmable array of OR gates. Devices are called PAL®-based when the Programmable Array Logic has a fixed OR array. The size (number of gates) of the programmable array and the size (complexity) of the highly configurable associated I/O (input/output) macrocells has been increased recently over that of the basic PLD architecture to meet the requirements of complex logic functions and high configurability. Recently, new kinds of PAL-based devices (chips) have appeared, such as the Altera Max family, AMD Mach family and XILINX EPLD devices that feature on each chip functional blocks which are structurally and functionally identical and have a programmable interconnect resource which interconnects the functional blocks. Such an interconnect is designated the programmable interconnect array (PIA) in the Altera Max family, the Switch Matrix in the AMD Mach family, and the Universal Interconnect matrix (UIM) in the XILINX XC7236 device. Each functional block on the chips mentioned includes a programmable AND array and several macrocells so that it is like a stand alone PAL, and upon appropriate configuration can be used logically separate from the rest of the chip or can communicate with the rest of the chip. Figure 1 illustrates in simplified form the architecture of the XILINX XC7236 chip, a PAL-baεed EPLD. Four configurable functional blocks FBI, FB2, FB3, FB4 are interconnected by a central universal interconnect matrix (UIM) . Each of the four blocks FBI - FB4 receives 21 input lines from the UIM, and connects by nine output lines to chip I/O pads labelled "I/O" and/or back into the UIM. Blocks FB2, FB3, FB4 receive 3 input lines coming from the I/O pads as well. Each block FB-1 - FB-4 has carry-in and carry-out lines as well as shift-in and shift-out lines, with the in lines of each block being connected to the out lines of a previous block and the out lines of each block being connected to the in lines of a next block, forming loops as illustrated around the blocks. Each of the functional blocks FBI - FB4 includes nine macrocells (not shown) and a programmable AND array (not shown) which is driven by 21 input lines from the UIM and three input lines coming directly from the I/O pads (for three blocks only in the case of the XC7236) . The XC7236 device thus contains 36 macrocells each having identical structure. Figure 2 shows the schematic diagram of one of the macrocells and the AND array present in each functional block of the XC7236 chip. Macrocells MCI - MC9 in each block (Figure 2 shows MCI only) are driven by the product terms (P-terms) derived from the programmable AND array AA2 in the same block. Five P-terms PP1-PP5, PP6-PP10, ... PP41- PP45 are private to each macrocell in the block, while an additional twelve P-terms SP1 - SP12 are shared among the nine macrocells in each block. Four of the private P-terms PPl - PP4 can be selectively (through programming of programmable switch elements SW1-SW4) logically ORed by gate OR1 together with up to four shared P-terms SP9 - SP12 and drive the Dl data input terminal of an arithmetic logic unit ALU. The other data input terminal D2 of the ALU is driven by the output signal of gate 0R2 which logically ORs the fifth private P-term PP5 and up to eight of the remaining shared P-terms SP1 - SP8. The four private P-terms PPl - PP4 can be programmed for other purposes; for instance the private P-term PPl can be used as a dedicated clock signal for the flip-flop FF; P-term PP2 can be the output enable signal OE; P-term PP3 and P-term PP4 can be the asynchronous SET S and RESET R signals for the flip-flop FF. The ALU can be programmed into two modes, logic mode and arithmetic mode. In the logic mode, the ALU is a 2- input function generator that can be programmed to generate any Boolean function of its two data input terminals Dl and D2. In the arithmetic mode, the ALU can be programmed to generate the arithmetic sum or difference of two operands, combined with a carry signal coming from the next lower order macrocell and feeds a carry output to the next higher order macrocell as well. The ALU output F drives the D input terminal of the flip-flop FF. Flip-flop FF can be programmed by configuration bit CB6 to be transparent, making the output signal on terminal Q identical with the D terminal input signal, independent of the clock input signal, or to operate in the conventional flip-flop manner, triggered by the rising edge of the clock input signal. The clock signal source is programmable by programming of configuration bits CB3 and CB4 to select (via multiplexer MUX2) either the dedicated P-term PPl, or one of two global clock signals FLCKO and FLCK1. The output signal on terminal Q of flip flop FF can be optionally fed back and logically ORed into the input terminal D2 of the ALU after being logically ANDed with three shared P-terms SP1 - SP3 by programming of the configuration bits CB1 and CB2. The polarity of the feedback signal can be controlled by the gate XOR by programming the configuration bit CB7. A logic block with macrocells of this kind is both complex and highly configurable and can implement a variety of logic functions by programming of the configuration bits such as CB1 - CB7, switches such as SWl - SW16, tri-state buffers such- as TSB1 and TSB2, and multiplexers in the ALU. However, such high configurability requires a large number of logic gates in the speed path, which causes additional propagation delay. This, together with the delay caused by routing signals through the UIM limits the speed of such devices. High speed and high flexibility of logic functions are generally in conflict with each other and must be traded off in such prior art devices. So far, no devices exist that can offer both high speed and high flexibility of logic functions for user applications. Some prior art FPGA devices include different kinds of blocks that are optimized for different applications. For example, the Actel ACT2 chip family has two types of logic blocks, a logic block for combinatorial functions and a logic block for both combinatorial and register functions. The two logic block types use common routing resources, and all blocks have their I/O routed via the common interconnect. These FPGAs do not have special sections of the chip with their own internal routing resources that optimize that particular section for speed or complexity. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a PLD having both high speed and logic function flexibility. Another object is to provide a hybrid architecture for a PAL-based chip which has sections optimized for speed and other sections optimized for logic function flexibility. The invention is based on the discovery that for most user applications, even in high speed user systems, only a portion of the PLD requires high speed, and the rest of the PLD tends to be used for applications that do not operate at high speed, but are logically complex. One example is a very fast state machine which controls a slower bus interface structure. Here, the state machine reaction needs to be very quick but the bulk of the logic does not need to be as quick. Another example is where an address decoder must be much faster than other logic. In such cases, a hybrid architecture is the optimum solution for such user application. According to the present invention, instead of a uniform structure with identical macrocells as in prior art EPLDs, the hybrid architecture includes two different kinds of PAL-like blocks, a fast functional block being optimized for high speed and a flexible block being optimized for logic function flexibility. In accordance with the present invention, the hybrid architecture includes on one chip functional logic blocks of a first type optimized for flexibility of logic functions, and blocks of a second type optimized for high speed. Preferably, certain of the high speed blocks are directly connectable to chip input and/or output pads (terminals) . Both types of cells include a programmable AND array, thus both types have some flexibility. Each fast block can selectively receive signals on a plurality of input lines directly from the chip I/O pads. This direction connection reduces the delay caused by the UIM. Also, output signals from the fast blocks can be directly fed back into that block without passing through the UIM. The fast blocks have dedicated output pads so as to increase speed. In one embodiment of the present invention, the fast blocks have a programmable AND array smaller in size (having fewer gates) than the AND array in the flexible blocks. Since a smaller AND array generates fewer product terms to the macrocells in the fast block, the fast block in this embodiment provides a faster AND array speed. The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the invention in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified architecture of a PAL-based XC7236 chip, which has uniform functional blocks. Figure 2 is a schematic diagram of one of the macrocells and the programmable AND array in each block of the XC7236 chip. Figure 3 illustrates a hybrid architecture optimized for both speed and high flexibility in accordance with the present invention. Figure 4 shows a schematic diagram of one of the macrocells and the reduced programmable AND array in each fast functional block of Figure 3. Figures 5 and 6 show schematic diagrams of output buffers optimized for flexibility and speed respectively. DETAILED DESCRIPTION OF THE INVENTION Figure 3 is a block diagram of an EPLD device in accordance with the present invention showing the hybrid architecture. (It is to be understood that Figure 3, like Figure 1, conventionally illustrates bus-type structures where one line on the figure or one circuit element represents respectively multiple conductors or multiple circuit elements.) The device includes ten identical conventional functional blocks FB2 - FB11 and two fast functional blocks FFBO and FFB1. Each conventional functional block FB2 - FB11 is a stand alone PAL, including a- programmable AND array and nine identical macrocells each similar to the ones shown in Figure 2, being complex in structure and highly configurable. Function blocks FB2 - FB11 each have a group of 21 input lines UI2, ..., uill from the UIM connected to the input lines of the functional block AND array, a group of nine output lines F03, ..., FOll connected from each block through a respective set of nine tri-state buffers TB2, TB3, ..., TB11 to a corresponding group of nine I/O pads Pd2, ..., Pdll. Further, a carry-in line is connected from terminal Cin in one block to terminal Cout in an adjacent block to form a loop as shown in Figure 3. The nine output signals from each block are fed back into the UIM through respective groups of lines FFB2, FFB3 ..., FFB11. "For each block, three input lines DI2, DI3, ... , Dill connect directly from an I/O pad to an input line of the AND array. These input lines are used when the I/O pads are used as input pads. Nine input lines, one from each of nine I/O pads of each functional block (for flexible blocks FB2, FB3, FB5 through FB8, FB10 and FB11) and a smaller number (for example 3) of input lines (for flexible blocks FB4 and FB9) drive the UIM via input circuitry IC2, IC3 ..., IC11, which can be programmed as an inverter, register or latch. As mentioned above, the functional blocks FB2 - FB11 each include highly flexible logic functions. Functional blocks FFBO and FFBl are in contrast fast functional blocks and feature faster speed rather than high flexibility. Fast functional blocks FFO and FF1 each include nine macrocells and a programmable AND array. Figure 4 is a schematic diagram of the AND array, one complete macrocell, and parts of adjacent macrocells in one of the fast functional blocks FFBO, FFBl, showing the carry logic between adjacent macrocells. As shown in Figure 3, twelve input signals coming directly from the twelve dedicated input pads Pdl can be selectively connected to the corresponding group of twelve input lines 10 and II of fast functional blocks FFO and FF1 by programming the associated sets of multiplexers IMUXO and IMUX1. The resulting data path does not pass through the UIM, providing a quicker input path. The other groups of three input lines UIO, UIl associated respectively with blocks FFBO, FFBl come from the UIM. The twelve input lines from the dedicated input pads Pdl and the three input lines from the UIM to each fast functional block FFBO, FFBl supply signals to the input lines of the programmable AND array AA4 of Figure 4. The fast functional blocks FFBO and FFBl each have nine dedicated output pads PdO and Pdl to which they are connected through associated output buffers BO, Bl and nine output lines OLO, 0L1 respectively. The nine output signals of each fast ' functional block FFBO, FFBl can selectively be fed back via lines FDBKO, FDBK1 into the same block directly by means of multiplexers FMUXO and FMUXl, providing a way of eliminating the delay otherwise caused by routing signals via the UIM. The nine output signals are also routed to the UIM, thus accessing the remainder of the device. As shown in Figure 4, the programmable AND array in each of the fast functional blocks FFBO, FFBl has a shorter data path length that the flexible function block of Fig. 2. For example, a signal on line 43 of Fig. 4 may have come from an input pad through a buffer, through an AND gate, through OR gate 41, through MUX 48, flip flop 47 (two gates) , inverter 66 to an output pad. This is a total of 7 gates. By contrast, in Fig. 2, an input signal from a pad will also pass through an input buffer and an AND gate. Then the signal will pass through a switch, an OR gate, an ALU (2 gates), a flip flop (2 gates) and an output buffer to a pad. This is a total of 9 gates. Thus the fast block saves two gates in the signal path. Compared to the high flexibility macrocell of Figure 2, the macrocell for the fast functional blocks is simplified in structure in order to achieve high speed. Macrocell 40 has five input lines 42 - 46 of P-terms from the AND array AA4. Four of the input lines 43 - 46 are the input lines to OR gate 41. The fifth input line 42 is a dedicated RESET signal line for flip-flop 47 when the output signal of OR gate 41 drives the D input terminal of flip- flop 47. Alternatively, input line 42 drives the D input terminal of flip-flop 47 when the output signal of OR gate 41 is cascaded to OR gate 52 in another (adjacent) macrocell 53 to expand the P-terms to be ORed by OR gate 52 in macrocell 53. The use of the signal on input line 42 depends therefore on the programming of configuration bit CBl. It is to be understood that Figure 4 shows only one example of a fast function block, and Figure 2 is only one example of a high complexity function block. As seen from the above description, macrocell 40 and hence the fast functional blocks FFBO, FFBl are optimized for high speed, while macrocell MCI and the corresponding functional blocks FB2, FB3, ..., FBll are optimized for logic function flexibility. With such EPLDs, the portion of a user design that requires high speed is implemented in the fast functional blocks FFBO, FFBl, while parts of the design that are logically complex and do not need to operate at high speed are implemented by functional blocks FB2 - FBll . This makes good use of the chip resources and provides both high speed and high flexibility for logical functions. In addition to offering a choice of flexibility versus speed in the macrocell, the present invention also offers a choice of flexibility versus speed in the output buffers. An output buffer preferably used when flexibility is important is shown in Fig. 5. This buffer offers an optional inversion of input signal DN as selected by one of transistors 56 or 58 which is turned on by signal CB or its complement CB. Output transistors Q51 and Q52 are sized to provide adequate drive and yet consume minimal power. For a higher speed output buffer, the circuit of Fig. 6 does not offer the optional inverter, thus shortening the signal path. In addition, higher speed is achieved by making the output transistors Q61 and Q62 wider. In one embodiment, the flexible output buffer of Fig. 5 uses transistor widths of 400μm and the high speed output buffer of Fig. 6 uses output transistor widths of 700μm. The output buffer of Fig. 5 is preferably used in the circuit of Fig. 3 for the three-state buffers TB2 through TBll and the output buffer of Fig. 6 is preferably used for buffers BO and Bl. However, in other embodiments higher output buffer speed can be combined with higher macrocell flexibility and higher macrocell flexibility can be combined with higher output buffer speed. Although the present invention has been described and illustrated with particular embodiments, it is to be understood that this is by way of illustration and example only and is hot to be taken by way of limitation. Different numbers of the two kinds of blocks, different circuitry of the two kinds of macrocells, and different input and output characteristics can be used, as apparent to those of ordinary skill in the art. The spirit and scope of the present invention is limited only by the terms of the appended claims.

Claims: CLAIMS I claim: 1. A programmable integrated circuit logic device comprising: a plurality of fast logic function blocks having input terminals at least some of which are connected to a pad of the device, and having at least one output terminal connected to a pad of the device; a plurality of flexible logic function blocks each having input terminals and at least one output terminal and designed such that more functions of signals on a given number of input terminals may be selectively provided at said output terminal of that flexible logic function block than can be provided by said fast logic function blocks; said fast logic function blocks being designed such that an output signal may be generated from input signals to that function block wherein the shortest sum-of-products path from input to output propagates through fewer gates than in said flexible logic function blocks; and an interconnect matrix for interconnecting at least some of said input and output terminals of said fast logic function blocks and said flexible logic function blocks to each other. 2. A programmable integrated circuit logic device as in Claim 1 in which said fast and flexible logic function blocks each include a programmable AND array. 3. A programmable integrated circuit logic device as in Claim 2 in which said fast logic function blocks each include an AND array less complex than an AND array in each of said flexible logic function blocks. 4. A programmable integrated circuit logic device as in Claim 2 in which said fast logic function blocks each further include a plurality of OR arrays each for generating a sum term from outputs of said AND array; and means for selectively providing an output of one of said OR arrays as input to another of said OR arrays, thereby expanding the number of inputs for which a sum can be generated. 5. A programmable integrated circuit logic device as in Claim 1 in which at least some of said input terminals of said fast logic function blocks can be connected directly to external pads of said device without passing through said interconnect matrix. 6. A programmable integrated circuit logic device as in Claim 1 in which at least some of said output terminals of said fast logic function blocks can be connected directly to external pads of said device without passing through said interconnect matrix. 7. A programmable integrated circuit logic device as in Claim 1 in which at least some of said output terminals of one of said fast logic function blocks can be connected directly to at least some of said input terminals of said one of said fast logic function blocks without passing through said interconnect matrix. 8. A programmable integrated circuit logic device as in Claim 1 in which said interconnect matrix is a universal interconnect matrix which can be connected to each of said flexible function blocks and said fast function blocks. 9. A programmable integrated circuit logic device as in Claim 1, wherein each flexible functional block comprises multiple first type macrocells of high logical function flexibility and a first type programmable AND array, and the fast functional block comprises multiple second type macrocells of high speed and a second type programmable AND array different from the first type AND array. 10. A programmable logic device including a plurality of blocks and programmable interconnects for interconnecting the blocks, comprising: first type functional blocks each having a first type programmable AND array and a plurality of first type macrocells each having a first number of logic gates; and second type functional blocks each having a second type programmable AND array and a plurality of second type macrocells each having a second number of logic gates less than the first number, wherein each of the second type functional blocks is programmably connected to a signal terminal of the device. 11. A device as in Claim 10, wherein the second type functional block programmably receives input signals directly from dedicated device input terminals. 12. A device as in Claim 10, wherein an output signal from the second type functional block is selectively fed back into said second type functional block. 13. A device as in Claim 10, wherein the second type programmable AND array has a smaller number of logic gates than does the first type programmable AND array. 14. A programmable integrated circuit logic device comprising: a plurality of logic function blocks each having at least one input terminal and at least one output terminal; a plurality of external pads; a plurality of output buffers of a first type each having a particular number of logic gates in its signal propagation path; a plurality of output buffers of a second type each having less than the particular number of logic gates in its signal propagation path; and an interconnect for connecting the output terminals of selected logic function blocks to said external pads by an output buffer, each said output buffer being one of the first and second type. 15. A programmable integrated circuit logic device as in Claim 14, wherein said output buffers of the first and second types each include an output inverter having two transistors, and wherein the two transistors in said second type of output buffer are each larger than the two transistors in said first type of output buffer.

Inventors: CHIANG DAVID, CHIANG, DAVID
Applicants: XILINX INC, XILINX, INC.
Application year: 1994
Main classification: H03K19
IPC codes: H03K19